440 BREAKDOWN OF GENETIC SYSTEMS 



embryos they must first fuse in pairs or fail to separate at mitosis. 

 Such a doubUng in the antipodal nuclei is characteristic of a section 

 of the related genus Tulipa (Ch. III). 



(c) Apospory, the development of cells other than the embryo-sac 

 cell into the gametophyte, is probably important in many species. 

 In Hieracium Ptlosella cells in the nucellus or integument may 

 develop into embryo-sacs, which, of course, are diploid (Rosenberg, 

 1906, 1930). These compete successfully with the reduced embryo- 

 sac in development and usurp its function. This is reminiscent of 

 the Renner effect (Ch. IX). Apospory is also found in Artemisia 

 nitida (Chiarugi, 1926) and in OcJina serrulata (Chiarugi and Francini, 



1930). 

 In all these species the normal embryo-sac has a chance of 



developing, either by fertilisation, as in Hier actum, or 



parthenogenetically, as in Artemisia and Ochna. The first is 



conditioned by the formation of a viable gamete in a plant which is 



Slightly sterile sexually owing to hybridity ; the second is conditioned 



by the formation of a diploid embryo-sac cell by the successful 



suppression of meiosis in a plant in which this suppression has not 



become a constant process. Apospory may then be said to be 



characteristically developed in those plants which are too hybrid 



to be regularly sexual or not hybrid enough to be regularly 



parthenogenetic. It bridges the gap. 



(d) Apospory with apogamy occurs in two ways. Nucellar 

 Emhryony is the purely vegetative development of cells in the 

 nucellus or integument into the embryo sporophyte. It gives 

 supernumerary embryos in sexually reproducing plants {e.g., Citrus, 

 Zygopetalum), and occurs as with parthenogenesis in Alchemilla, 

 Nothoscordon fragrans, and Ochna " multiflora," which are sexually 

 sterile. 



Vivipary is the replacement of the flower by a purely vegetative 

 outgrowth, which is capable of propagating the plant as a bulbil. 

 It is principally developed facultatively and then shows considerable 

 variation subject to environmental influence. It is found in Allium, 

 Poa, and Festuca, especially in sexually sterile species, such as 

 triploids and auto-tetraploids (Levan, 1933 a ; Miintzing, 1933)- 

 The imposture is recognisable at once from the external morphology 



